1. Field of the Invention
This invention relates to an optical disc recording apparatus and an optical disc reproducing apparatus enabling Tong-time recording and long-time reproduction of audio signals. More particularly, it relates to an optical disc recording apparatus and an optical disc reproducing apparatus enabling continuous recording and reproduction of audio signals on or from plural optical discs.
2. description of the Prior Art
The optical disc has a recording capacity two to three orders of magnitude higher than that of a magnetic disc, while enabling high-speed access as compared to a tape-shaped recording medium. Besides, the optical disc is excellent in durability, while enabling non-contact data recording/reproduction. For these reasons, the optical disc has come to be used extensively in recent years. One of the most well-known forms of the optical discs is a compact disc (CD).
For providing portable, above all, pocketable size headphone stereo devices, with the use of the optical disc, a CD with a disc diameter of 12 cm and a CD with a disc diameter of 8 cm (so-called CD single) have been proposed. Since the recording/reproducing apparatus is increased in size and hence is not readily portable, a disc with a diameter of 8 cm or less would be preferred. However, in attempting to construct a portable or pocketable size recording/reproducing apparatus for an optical disc 8 cm or less in diameter, the following problem arises.
In the case of a standard CD format optical disc, on which are recorded stereophonic digital PCM audio signals sampled with a sampling frequency of 44.1 kHz and quantized with 16 bits, and where these signals are exclusively reproduced by the user (CD-DA format), the reproducing time of the disc 8 cm in diameter is 20 to 22 minutes at most, so that a symphony, for example, cannot be recorded on one disc side. A playback time of 74 minutes or longer, which is approximately equal to that of a 12 cm CD, is preferred. Besides, it is not possible for the user to make recording with this CD-DA format.
On the other hand, with a CD-MO format (a format employing a recordable magneto-optical disc) as an extension of the above-mentioned standard CD-DA format, the recording/reproducing time of the 8 cm disc is only 20 to 22 minutes, as in the CD-DA format.
With the CD-I (CD-Interactive) format, levels A to C shown in Table 1 are prescribed as the modes for recording/reproducing bit-compressed digital audio signals.
TABLE 1 ______________________________________ NUMBER PLAYBACK SAMPLING OF TIME FRE- QUANTIZA- BAND- STEREO/ LEVEL QUENCY TION BITS WIDTH MONO ______________________________________ A 37.8 kHz 8 17 kHz 2/4 B 37.8 kHz 4 17 kHz 4/8 C 18.9 kHz 4 8.5 kHz 8/16 ______________________________________
When reproducing a disc recorded with e.g. the level B mode, signals obtained by quadruple bit compression of standard CD-DA format digital signals are reproduced. Thus, if all of the recorded data are compressed stereophonic audio data, playback time is quadrupled, or reproduction of 4-channel data becomes feasible, so that reproduction for 70 minutes or longer becomes possible with an optical disc 8 cm in diameter or less.
With the CD-I format, the disc is rotationally driven at the same linear velocity as that for the standard CD-DA format, so that continuous audio compressed data are reproduced at a rate of one unit to an recorded units on the disc, where n Cs a figure corresponding to the playback time or the compression rate of data, and is equal to 4 with the level B stereo mode. This unit is termed a block or sector, which Cs made up of 98 frames and has a period of 1/75 second. Therefore, with this level B stereo mode, a data string in which every fourth sector is an audio sector, such as: EQU S D D D S D D D . . . .sigma.
where S is an audio sector and D is another sector (s), is recorded on the disc. However, for actual recording, since the above data string is processed with predetermined encoding similar to that for ordinary CD format audio data, such as error correction coding and interleaving, data of the audio sector S and data of the data sector D are arranged in a scrambled fashion in the recording sectors on the disc. The other data sectors may, for example, be video or computer data. When bit-compressed audio signals are also used for the data sector D, a data string in which four-channel audio sectors S1 to S4 are cyclically arranged; that is a data string: EQU S1 S2 S3 S4 S1 S2 S3 S4 . . . . . .
is encoded and recorded on the disc. When recording and reproducing continuous audio signals, the above-mentioned 4-channel audio signals are linked sequentially beginning at the first channel and terminating at the fourth channel. More specifically, channel 1 data corresponding to the audio sector S1 are reproduced from the innermost to the outermost areas of the disc. Channel 2 data corresponding to the audio sector S2 are reproduced from the innermost to the outermost areas of the disc. Channel 3 data corresponding to the audio sector S3 are reproduced from the innermost to the outermost areas of the disc. Finally, channel 4 data corresponding to the audio sector S4 are reproduced from the innermost to the outermost areas of the disc to enable data reproduction for a continuous quadrupled time duration.
However, for the above-mentioned continuous reproduction, several track jump operations spanning the inner and outer disc peripheries are required. Since a track jump cannot be performed instantaneously, playback data are interrupted for a short time period. This means that the reproduced sound is interrupted momentarily.
For overcoming the above-described problem, the present Assignee has proposed the following technique.
More specifically, when data similar to those of the above-mentioned CD-DA format, that is audio PCM data having a sampling frequency of 44. 1 kHz, 16 quantization bits and a data transfer rate of 75 sectors/second, are recorded after bit compression in accordance with the B-level stereo mode, the sampling frequency of the audio PCM data with the data transfer rate of 75 sectors/second is converted to 37.8 kHz, while the number of quantization bits is reduced to 4, to produce ADPCM audio data with a data transfer rate equal to one-fourth the original rate, or 18.75 sectors/second. The B-level stereo mode ADPCM audio data, continuously outputted at the rate of 18.75 sectors/second, are written in a buffer memory. When the data volume of the ADPCM audio data stored in the buffer memory exceeds a predetermined volume K, the ADPCM data are read out from the buffer memory as recording data by the predetermined volume K in a burst fashion at a transfer rate of 75 sectors/second. These data are recorded in a continuous state on a recording track of the magneto-optical disc by address-controlling the recording positions on the recording track.
That is, in the above-mentioned recording system for the ADPCM audio data, the ADPCM audio data are continuously written in the buffer memory at a transfer rate of 18.75 sectors/second, by continuously incrementing the write pointer of the buffer memory at the transfer rate of 18.75 sectors/second. When the data volume of the ADPCM audio data stored in the buffer memory exceeds the predetermined volume K, the Predetermined volume K of the ADPCM audio data are read out from the buffer memory as the recording data in a burst fashion at the above-mentioned transfer rate of 75 sectors/second by incrementing the read pointer of the buffer memory in a burst fashion at the transfer rate of 75 sectors/second.
On the other hand, in the reproducing system for the ADPCM data, playback data from the disc are written into the buffer memory at the transfer rate of 75 sectors/second by incrementing the write pointer of the buffer memory at the transfer rate of 75 sectors/second, while the playback data are continuously read from the buffer memory at the transfer rate of 18.75 sectors/second, by continuously incrementing the read pointer of the buffer memory at the transfer rate of 18.75 sectors/second. Writing is discontinued when the read pointer is overtaken by the write pointer. When the data volume of the playback data stored in the buffer memory is lower than a predetermined volume, the write pointer of the buffer memory is incremented in a burst fashion at the transfer rate of 75 sectors/second to again write data i n the buffer memory.
By controlling the buffer memory in this manner, the B-level stereo mode ADPCM audio data reproduced from the recording track of the magneto-optical disc are written in the buffer memory in a burst fashion at the transfer rate of 75 sectors/second, while the ADPCM audio data are continuously read from the buffer memory as playback data at the transfer rate of 18.75 sectors/second. In this manner, the playback data are continuously read from the buffer memory while the data volume exceeding the predetermined volume is maintained at al 1 times in the buffer memory. By address-controlling the playback position on the recording track of the magneto-optical disc, the audio data may be continuously reproduced from the recording track of the magneto-optical disc.
If it is attempted to record continuous audio signals, it is not possible to record only the sector S2 signals, by reason of the interleaving operation performed at the time of recording, but it becomes necessary to perform interleaving involving neighboring sectors S1 and S3 or even the sectors neighboring thereto, such that it becomes necessary to rewrite signals of the pre-recorded signals. This indicates that it is extremely difficult to record the compressed continuous audio data.
If it is possible to switch between the above-mentioned compression modes, the usage of the recording/reproducing apparatus may be enhanced advantageously. However, if the rotational speed of the disc, recording pattern or signal processing operations should be switched for each of these selected compression modes, the construction of the circuit is complicated to render it difficult to render the apparatus inexpensive. It is therefore desirable that the controlling or signal processing operations or the recording patterns be changed to the least extent with changes in the compression modes.
For overcoming the above-mentioned problem, the present Assignee has proposed a technique in which digital data are arranged into clusters at an interval of a predetermined number of, for example, 32 sectors, and e.g. five cluster-linking sectors, each longer than an interleaving length, are provided at each cluster-linking region so that data interleaving in each cluster does not affect the neighboring clusters.
Long-time recording or long-time reproduction of the audio signals becomes possible by performing bit compression of the digital audio signals before recording these digital audio signals. However, the recordable data volume on one optical disc is limited so that the maximum recording time is improved by the recording capacity of the optical disc. Although continuous recording or continuous reproduction for an extended period of time becomes possible by using plural discs one after another, signal dropout of the audio signals is produced at the time of exchange of the optical discs.